Power generation technologies and related systems, including gas turbine engines, jet engines, wind turbines and related platforms or towers, are often subject to dynamic operating conditions. The potential stresses of such operating conditions, particularly high temperature and pressure conditions, require power generation components having increased strength and durability. One exemplary class of materials that has been developed for use in such environments corresponds to superalloys.
Superalloys are alloys containing about 50% or more by weight of a base metal, including but not limited to nickel, cobalt and iron, to which alloying elements are added to improve the mechanical and physical properties of these alloys. One particular example of a suitable superalloy for aircraft and industry gas turbine components and other applications is René N5, a Nickel-based rhenium single crystal superalloy. These superalloy materials have been found to exhibit not only good strength, but also creep resistance, fracture roughness and other mechanical properties at elevated temperatures for extended periods of time.
The welding together of superalloy materials has been a relatively difficult process requiring very particular welding conditions. For example, the use of a low heat input welding process, such as laser or electronic beam, has produced weld joints over a very narrow range of welding conditions. One drawback to these beam processes is the directional grain growth in the fusion zone which forms a distinct dendritic boundary in the center of the weld zone. This type of grain structure makes the joint vulnerable to centerline cracking and results in very poor fatigue strength, which can result in catastrophic failure of the weld joint during operation of a gas turbine.
To overcome the centerline cracking problems, several alternative processes have been developed for welding superalloys. Among them, the wire feed electron beam process, autogenous laser welding, gas tungsten are process (TIG), and preplaced shim electron beam or laser processes have been considered in the context of improving fatigue life of the joint. A simple wire feed welding process adds ductile superalloy filler metal, through an automatic wire feeder during electron beam welding of two metallic pieces. However, this process is limited by the joint thickness. Also, lack of penetration (LOP) defects often occur when the joint thickness is increased beyond 0.25 cm. Laser based welding without the use of a filler metal (i.e., autogenous welding) can exhibit very low ductility and can crack during, or soon after, solidification. The high heat input associated with are welding can cause relatively large airfoil distortions and increase the risk of lack of fusion defects in the weld, thus prohibiting use of TIG techniques as the primary welding process for complex airfoil structures. Adding a pre-placed shim between two welded components has increased joint thickness as well as the ductility of the weld deposit to reduce the cracking of the weld metal. However, cracking may still occur if the ductility is not high enough.
The art is continuously seeking improved systems and methods for welding superalloys and other materials, to improve the performance of welded components and expand the repair options related to use of such components.